The surprisingly powerful role of fear
An unpleasant emotion caused by the belief that someone or something is dangerous, likely to cause pain, or a threat: — Oxford English Dictionary
Fear is an emotion induced by a perceived threat which causes entities to quickly pull far away from it and usually hide. — Wikipedia
To be afraid of (something or someone). To expect or worry about (something bad or unpleasant). To be afraid and worried. — (Not very helpful) Merriam-Webster.com
Both Meredith and I have talked a bit about the meaning and role of “fear” in shaping animal behaviors and population dynamics. The word “fear” is a bit touchy. When ecologists use the word fear, we aren’t talking about the emotion as you and I know it. We are referring to a certain type of situation and response. For example, lions kill and eat wildebeest. This creates “landscape of fear” – meaning that the wildebeest exists in a landscape in which certain physical places have a higher risk of predation. You can envision that this landscape has its own topography — hills and valleys of high and low risk. The differing levels of risk can trigger physiological responses as well as behavioral responses. For example, wildebeest may show higher levels of stress hormone in the “risky” areas, or they may avoid “risky” areas even though that’s where the best food is.
This is what we mean when we talk about fear. We are not talking about whether the wildebeest lies awake at night dreaming bad dreams. We are talking about situations of high and low risk and the physiological and behavioral responses.
That being said, “fear” is an incredibly powerful driving force in the natural world. I’ve touched on this from time to time. The idea that smaller predators are so desperate to avoid being beaten up by the big guys, that they avoid the areas with the best food or den sites, and their populations decline even if they aren’t actively being killed by the big guys. This process still amazes me. Even cooler? Fear doesn’t just matter for big and small predators, it doesn’t just matter for predators and prey. The effects of fear can trickle down from predators to prey to plants, just like the trophic cascades I wrote about last week.
Some of my favorite research on the role of fear in trophic cascades has been done by researchers out of the Schmitz Lab at Yale University.
In 1997, Os Schmitz and his students hypothesized that predators could trigger trophic cascades not just by killing and eating herbivores, but by scaring herbivores and changing their behaviors. Os, in his infinite wisdom, works in systems that are experimentally tractable. So he and his team got a bunch of spiders (their predator) and grasshoppers (their prey) and did an experiment that I will never ever be able to do with lions. They created two treatments: a risk treatment, where the spiders had their pincers glued shut and couldn’t kill the grasshoppers, and a predation treatment, where the spiders got to carry on with all the spidery things they like to do (such as eat grasshoppers). They put the grasshoppers in with one of the two types of spiders, and compared what happened.
So, perhaps unsurprisingly, grasshoppers were afraid of spiders whether or not the spiders had their mouths glued shut. In the presence of any spider, grasshoppers changed their diet to avoid areas that spiders liked to lurk, spent less time eating, and only really came out to eat when the spiders were sleeping. The surprising thing is that these behaviors resulted in lower grasshopper densities irrespective of whether or not the spiders could kill grasshoppers. The presence of spiders with their mouths glued shut changed the behavior of the grasshoppers, which resulted in the grasshoppers acquiring less food, which in turn decreased grasshopper populations. What’s more, these effects trickled down to the plant communities. Grasshoppers eat grass, but mere presence of predatory spiders can reduce the effect of grasshoppers on this grass.
Since this 1997 experiment, Os’s lab has gone on to produce some of my favorite research on the role of fear in driving ecological systems. Now if only I could figure out how do such enlightening experiments in the Serengeti…
Reference: Schmitz, O.J., Beckerman, A.P. & O’Brien, K.M. (1997) BEHAVIORALLY MEDIATED TROPHIC CASCADES: EFFECTS OF PREDATION RISK ON FOOD WEB INTERACTIONS. Ecology, 78, 1388–1399.